The present invention regards a circuit for Single Photon Avalanche Diodes (SPAD) to be used in conjunction with an avalanche quenching circuit for high precision detection of the time of arrival of the photon, that is the instant in which the single photon hits the active surface of the detector. The invention in particular regards the field of the technique of Time Correlated Photon Counting, (TCPC) and more generally the field of the techniques that use precision measurements of the time of arrival of photons, such as distance measurements with laser ranging techniques.
Time Correlated Photon Counting TCPC techniques are used for measuring fast and/or weak optical signals in various technical and scientific fields (study of fluorescent emission and decay in science of materials, chemistry, biology, medicine, etc.). Similar techniques based on precision detection of time of arrival of photons are used in various other fields such as: satellite laser ranging; measurement of shape of remote objects with optical radar techniques; quantum cryptography. The time resolution that can be obtained with said techniques is determined by the precision with which the arrival instant of the incident photon on the photodetector is identified. Many applications require to work also with extremely high counting rates (Mc/s, millions of counts per second) keeping the good level of time resolution obtained at low counting rate (kc/s, thousands of counts per second, or less).
The said techniques have been introduced and developed using Photo-Multiplier Tubes (PMT) as detectors of single optical photons.
Special semiconductor devices, single-photon avalanche detectors (SPAD) are at present known and available as detectors of single optical photon. In comparison to the PMTs the SPADs represent a definite progress thanks to their smaller dimensions, lower bias voltage and power dissipation and to their compactness, ruggedness and reliability. Furthermore, they provide higher quantum efficiency and a precision in the detection of arrival time photons, which is comparable with that of the best Micro Channel Plate (MCP) photo-multipliers.
The single photon avalanche detectors are special avalanche photodiodes operating in Geiger mode biased at a bias voltage VAA higher than the breakdown voltage Vbd, that is with an excess bias voltage Vex=|VAA|xe2x88x92|Vbd| greater than 0. At said voltage, a single photon that by hitting the detector frees an electron-hole pair can trigger a self-sustaining process of avalanche multiplication of the charge carriers. The photon thus produces a macroscopic current pulse (typically one milliAmpere or more) with a fast leading edge (typically less than a nanosecond). Said pulse signals the arrival of the photon and indicates with its leading edge the instant of its arrival.
The avalanche current pulse needs to be terminated in order that the device be able to detect other photons in following times. A quenching circuit accomplishes this task by lowering the voltage applied to the photodiode down to the breakdown voltage Vbd or below it. Therefore, in correspondence with a single photon detected, the SPAD produces a current pulse of short duration (typically from ten to some tens of nanoseconds).
Passive Quenching Circuits PQC and Active Quenching Circuits, AQC are known. In the passive circuits PQC the quenching is caused by a high value load resistance, on which the avalanche current itself directly develops the voltage drop that quenches it. In the active circuits AQC a special circuit block (AQB Active Quenching Block) including active devices detects the triggering of the avalanche current and applies to the SPAD a voltage pulse, which lowers the voltage applied to the SPAD down to below the breakdown voltage Vbd.
The main features of the known SPADs reported in the technical and scientific literature can be summarized as follows. Depending on their structure, they can be classified in two categories: thin junction SPADs, by means of which the best time precision has been reached (up to 20 ps picoseconds), that have junction thickness 1 micrometer or a little more, low breakdown voltage (in the range between 15 and 50 V), excess bias voltage Vex up to 10 V or a little more; thick junction SPADs, by means of which a lesser time precision has been reached (up to 150 ps), that have typically a thickness of the junction of 20 micrometers or more, high breakdown voltage (in the range of 120 V to 500 V), excess bias voltage Vex up to some tens Volt. The reported SPAD devices have a photon sensitive area with a diameter in the range from 5 to 500 micrometers, surrounded by a large guard ring not sensitive to photons, which contributes to the electrical capacitance of the junction, that has values in the range between 1 picofarad and about twenty picofarad. When the avalanche current flows, the SPADs have an internal resistance Rd whose value depends on the structure of the device and is in the range from a few hundred Ohm to about 10 Kohm. The value of the avalanche current is given by the ratio between the excess bias voltage Vex and the internal resistance Rd of the SPAD. The risetime of the avalanche current pulse is very fast, and its value depends on the structure of the SPAD and on the value of the excess bias voltage Vex: typically it is shorter than one nanosecond, it can be as low as a hundred picoseconds or it can be longer up to few nanoseconds. From hereon the bandwidth limit of a pulse with risetime Tra will be defined as the reciprocal 1/Tra of said raise time. For the avalanche current pulse of the SPADs the bandwidth limit is typically greater than 1 GHz and it can be higher, up to a few GHz, or be reduced, down to some hundred MHz.
The object of the present invention is to make an output circuit which can be used in conjunction with quenching circuits of various types for operating with any type of SPAD at any bias voltage (VAA voltage even higher than 500 V) and with any pulse counting rate (even high counting rate exceeding 1 Mc/s one million counts per second), which permits to extract the avalanche current signal in such a way that in any operative condition it is possible to identify and measure with high precision the instant of the avalanche triggering, and therefore the instant in which the photon arrives on the active surface of the SPAD.
In accordance with the present invention, said object is reached by means of a quenching circuit and output circuit for a SPAD photodiode suitable for producing a signal with a risetime of the leading edge almost as rapid as the intrinsic risetime of the leading edge of the avalanche current within the SPAD, therefore in a time range from a few tens of picoseconds to some nanoseconds and having a total duration in a time range from a few nanoseconds to a few tens of nanoseconds, said circuit comprising a comparator for detecting the avalanche pulse that has input connected to an output point of a circuit coupled in alternate current (AC-coupled), which has its input point connected to a first terminal of the SPAD photodiode having the bias voltage applied to it, and means placed between the ground and the second terminal of said SPAD for quenching the avalanche and taking out and counting signals, is characterized in that in said circuit block the circuit elements that constitute it are such to determine a filtering action with a low-pass type cut-off on the high frequency side, with the characteristic cut-off frequency preferably corresponding to a simple pole, that is to a simple integration time constant, and on the low frequency side a high-pass type cut-off, with the characteristic cut-off frequency preferably corresponding to a simple pole, that is to a simple differentiation time constant, and in that the values of the circuit elements are selected so that the value of said high-pass cut-off frequency is less than said low-pass cut-off frequency and less than the bandwidth limit of the avalanche current pulse, but is greater than the value determined by the reciprocal of the duration of said pulse, that is the said differentiation time constant is greater than that of the integration and greater than the rise-time of the avalanche current pulse, but is less than the duration of said pulse.
Furthermore the values of the circuit elements are preferably selected so that the value of said low-pass cut-off frequency is preferably greater than the bandwidth limit of the avalanche current pulse or at least about equal to it, that is the said integration time constant is less than the risetime of the avalanche current pulse or at least about equal to it.
The above mentioned value of the low-pass cut-off frequency and of the corresponding integration time constant is established in order to prevent that the slowdown, that the rate of rise of the leading edge of the output pulse suffers with respect to that of the avalanche current pulse, be significant enough to degrade the precision of the measurement of the arrival time, as is herewith clarified. The above mentioned value of the high-pass cut-off frequency and of the corresponding differentiation time constant is established in order to prevent that at high counting rates the baseline of the output pulses be affected by statistical fluctuations, with a consequent degradation of the precision in the measurement of the arrival time of the photons, as is herewith clarified.